1. Field of the Invention
The present invention relates generally to the field of color gamut computation in color reproduction and, in particular, the invention relates to a system for efficiently and accurately computing a color gamut for any medium expressing color and in which the channel intensities can be controlled digitally.
2. Description of the Related Art
This invention is in the field of color reproduction in general and, in particular related to the explicit determination of the color gamut for a medium typically employed in a color process such as an output process like ink or dye on paper or exposure of film. Examples of such processes are:
offset lithography, in which a four-color process employs printing inks as colorants applied to paper; PA1 halftone screen pattern, by a printing press; PA1 gravure printing which employs printing inks applied to paper; PA1 off-press proofing systems which employ toners as colorants to simulate the effect of an offset press; PA1 computer-driven printers, which use a variety of technologies and colorants, such as jettable inks, toners, and dyes, applied in various continuous-tone, halftone, or dithered patterns; or digital exposure of negative or positive films by a film writer. PA1 an input transform mapping the native device color space to the PCS; PA1 an output transform mapping the PCS to the native device space; PA1 a simulation transform which maps the PCS to PCS and describes how out-of-gamut colors are printed on the output device; and PA1 a gamut alarm transform which maps the PCS to a single number and indicates whether or not a given color is in gamut or out of gamut.
Most of the paper-based processes make use of three chromatic colorants (inks, dyes, toners) while the film-based processes are controlled by the intensity of red, green and blue light exposures. For paper-based processes the colorants are commonly chosen to be the subtractive primaries cyan, magenta, and yellow (abbreviated as C, M, and Y). In addition, there may be an achromatic, or black, colorant (abbreviated as K) which increases the overall range of darker colors attainable.
To print digital images on these devices, it is often necessary to convert colors among different device color spaces. Consider the workflow in which a picture is digitized by a scanner, viewed on a monitor and then output to a four-color offset press. To obtain a pleasing rendering in all steps of this process, it is necessary to have explicit control over the transformations which convert among all the various color spaces: 1) scanner RGB, 2) monitor RGB, and 3) CMYK inks on press.
Color management systems have been developed by various companies which perform the above outlined task, namely, to produce desired and pleasing renderings of images from input, display and output. These color management systems use device profiles to describe the colorimetric properties of color capable devices with respect to the mediums they utilize. Each profile contains transformations between the native device color space and a device independent color space, also referred to as the profile connection space (PCS). Designated color spaces for the PCS are CIELAB and CIEXYZ defined by the International Commission on Illumination (CIE). In addition, profiles for output devices contain a simulation transform which maps from PCS to PCS which in essence describes how out-of-gamut colors are mapped onto the gamut of the output device. This permits a simulation of the output to be displayed on the monitor. In summary, then, each output profile contains the following transforms:
The format for these profiles has been standardized by the International Color Consortium (ICC). A number of companies have produced and sold profiles which conform to this standard, as well as software applications which generate conforming profiles from measured data.
The generation of all of the above listed transforms within a profile is facilitated by having knowledge of the color gamut of any color capable device in the workflow chain. Furthermore, the ICC standard is increasingly being accepted by the color reproduction industry as a way of attaining portable color communication across different devices and computer platforms. As a result, the ability to efficiently and accurately generate color gamuts, which is an important component in characterizing any color capable device, has taken on new importance.
In the past the gamut of a color device, such as a printer, has been developed by measuring the color outputs of the printer using color signals spaced at regular intervals in the channels of the device space, developing a forward model that describes the color behavior of the device when colors are transformed from the device color space to the PCS, inverting the model at each quantized L* and theta value in the PCS and finding the color of greatest chroma which represents the gamut boundary (see for example U.S. Pat. No. 5,563,724). This approach has two practical problems. The first is that the inversion of the forward model is computationally intensive making gamut construction or mapping impractical for real time applications. What is needed is a system that is less computationally intensive and is suitable for real time applications.
The second problem relates to the fact that theta quantization on an L* plane is linear, i.e. the differences between each bin is constant. Linear quantization increases the likelihood that important gamut features will not be accurately described due to quantization error. Because of this problem the most saturated colors in the gamut can be missed. This problem can be minimized by increasing the quantization in theta but at the cost of increased computational time and disk storage requirements. What is needed is a system that describes the gamut while still being computationally fast.